In this work we report on bulk Si-Al-O-C ceramics produced by pyrolysis of commercial poly(methylsilsesquioxane) precursors. Prior to the pyrolysis the precursors were cross-linked with a catalyst, or modified by the sol-gel-technique with an Al-containing alkoxide compound, namely alumatrane. This particular procedure yields amorphous ceramics with various compositions (Si1.00O1.60C0.80, Si1.00Al0.04O1.70C0.48, Si1.00Al0.07O1.80C0.49, and Si1.00Al0.11O1.90C0.49) after thermal decomposition at 1100°C in Ar depending on the amount of Al-alkoxide used in the polymer reaction synthesis. The as-produced ceramics are amorphous and remain so up to 1300°C. Phase separation accompanied by densification (1300-1500°C) and formation of mullite are the stages during heat-treatment. The nano-crystals of mullite are embedded in an amorphous Si(Al)OC matrix. An activation energy of 503 ± 60 [kJ/mole] equivalent to mullite crystallization is calculated, which is similar to monophasic gels. A minimum oxidation rate constant of 6.142 x 10-2 [mg2/cm4 x h] is calculated for the oxidation of pyrolyzed aluminum modified ceramic at 1400°C. The change in the viscosity and activation energy with aluminum content is investigated by the creep behavior of the SiAlOC ceramics. A glass transition temperature of 1220-1300°C similar to SiOC ceramic is mentioned and decreases with increasing content of aluminum in the material. Potential application of polysiloxane derived SiOC ceramic in the field of ceramic micro electro mechanical systems (MEMS) is reported.